Recently I purchased a Eurorack VCO, which shall remain unnamed.  When testing it out, I turned the coarse frequency knob all the way up to see how high it would go.  It quit oscillating!  I contacted the manufacturer, who explained that this was normal behavior for that VCO.  It was only good up to about 10 Khz.  I returned the module.  But this got me thinking.  How normal was this behavior for a VCO to stop oscillating at a certain frequency?  I decided to conduct a test of most of my VCOs, including some self-oscillating filters.  I’m dividing the following into two sections, the first for 5U MOTM style and BugBrand, and the second for Eurorack.  They are listed from the highest to the lowest maximum frequency.  The procedure was to turn up the coarse and fine frequency knobs all the way, and then add up to +5V to frequency CV inputs.

MOTM 5U + BugBrand

1. Teezer

The hands down winner is Ian Fritz’s Teezer through-zero VCO.  It is capable of greater than 100 Khz, and does not lose output amplitude or shape.  Pictured is the sawtooth output at a bit over 100 Khz.  My DMM cannot count higher than 99.99 Khz, so it reads OL.

2. MOTM-300

The MOTM-300 Ultra VCO came in second at 95.53 Khz.  At this frequency, the sine wave output is significantly attenuated, so I looked at the square output, which as you can see is slewed.

3. MOTM-440 VCF

The 440 puts out a clean sine wave, shown here at 60.76 Khz.

4. BugBrand SYN2A VCO

The SYN2A VCO has Tri/Saw and Square/Pulse outputs.  Here is the triangle output, only slightly distorted at 40 Khz.

5. Mattson VCF

George Mattson’s VCF self-oscillates nicely.  Here it puts out an sine wave at 38.39 Khz.

6. Buchla 258J Clone

Here’s the sine output from one of my 258J clones, at 36.63 Khz.

7. MOTM-490 VCF

Here is the 490, self-oscillating at 30.04 Khz.

8. BugBrand SYN2B Quadrature Sine VCO

The SYN2B can go up to 28.4 Khz.

9. Oakley Super Ladder VCF

The Super Ladder VCF self-oscillates up to 28.21 Khz.

10. Cynthia ZerOscillator

The ZO goes up to at least 20.71 Khz.

11. MOTM-420 VCF

The 420 can self oscillate, producing a clean sine wave at 20.52 Khz.

12. Modcan VCDO

The VCDO reached up to 18.40 Khz.  The sine wave looked a little fuzzy, even after a double-check on the scope focus.  Probably due to it being digitally synthesized.

13. Synthesis Technology e340 and e350.

The e340 and e350 are Eurorack VCOs that I converted to 5U panels.  So I put them in this group.  They both measured the same, which is not surprising, because they use the same PC board.  These max out at about 10.15 Khz.  But they don’t quit if you give them more CV.

14. MOTM-320 LFO

My 320 LFOs were modified to have a high frequency toggle switch.  It just adds more positive voltage to the frequency base.  I was able to get 8.59 Khz out of one of them.  The other, slightly less.

15. CGS Bi-N-Tic Filter

The Bi-N-Tic filter has a built-in VCO.  Mine has four sub-octaves available as square waves.  This measurement used the first sub-octave, at 2.98 Khz, meaning that the oscillator was running at about 6 Khz max.  Adding more CV did cause the VCO to quit oscillating.  This was the only one in this group that quit.

16.  XR-2206 DIY VCO

The XR-2206 chip in this experimental VCO is able to go as high as 4.31 Khz.

Eurorack Oscillators

1. Ieaskull F. Mobenthey Denum VCO

The highest VCO of the Eurorack group is the Denum, by Peter Blasser.  Here’s the square wave output at 56.23 Khz.  The square is slewed into a virtual sine wave.

2. Noise Engineering Sync Iter

The Sync Iter is billed as a wide range VCO, and indeed it is.  Here is the bi-polar output, in plain mode, with morph set at zero.  It gets 46.11 Khz, with an oddball wave shape.

3. Make Noise STO

The STO can put out a clean sine wave at 30.51 Khz.

4. Synthesis Technology e950 Circuit Bent VCO

The e950 can oscillate at 26.04 Khz.  Adding more CV makes it unstable, but it doesn’t quit.  The amplitude drops off over about 10 Khz.

5. Ieaskull F. Mobenthey Fourses

Here is one of the four Fourses oscillators, with its bounds defined.  It reached 22.63 Khz.  The triangle is smoothed out, and it is offset into the negative voltage region.

6. WMD PDO

The PDO went as high as 20.60 Khz.  This was below the maximum rotation of the coarse frequency knob.  Turning the knob further resulted in erratic output and even complete halting of the oscillation.  Pictured is the square output.  I wasn’t able to sync the scope on it.

7. Noise Engineering Loquelic Iteritas

The L. I. went up to 12.20 Khz.  I selected the square wave to look at.  L. I. remains stable if more CV is added.

8. Mutable Instruments Warps

Warps has an internal VCO, which can be accessed.  I was able to push the frequency up to 12.13 Khz on this digital VCO.  This is the sine wave, obviously.

9. Mannequins Just Friends

It’s hard to describe Just Friends, but it can function as a VCO.  I was able to get 11.78 Khz out of it.  But adding more CV makes it unstable.  I don’t know why the waveform is fuzzy.

10. Mutable Instruments Braids

Braids is a digital oscillator.  I expected it might go higher, but was only able to get 8.897 Khz.  Pictured is the CSAW algorithm.  It has morphed away from a sawtooth shape.

11. Mutable Instruments Tides

Here is Tides, producing a sine wave at 7.789 Khz.